This invention relates to an improved dye penetrant composition for non-destructively testing materials to locate defects open to the surface, such as cracks, and is more particularly concerned with the provision of a biodegradable dye penetrant composition which is extended with a biodegradable extender which is compatible with the liquid vehicle of the dye penetrant, is highly water soluble, and is an excellent solvent for dyes and surfactants used in dye penetrant formulations, as well as possessing other important characteristics such as low viscosity, low volatility, quick penetration into cracks and voids, low toxicity, and low metal corrosion.
In known penetrant inspection methods for rapid location and evaluation of surface flaws such as cracks in test bodies or parts, a dye penetrant composition, preferably containing a fluorescent dye, and which will penetrate the openings of the surface of cracks or other voids in the part, is applied to the surface of the test body, and the excess penetrant composition is removed from the surface of the body. A developer composition may then be applied to the part surface, which acts as a wick and causes the liquid penetrant containing the fluorescent dye, which was retained in the cracks, to be drawn up out of the surface defects by capillary action. The part is then exposed to appropriate lighting conditions such as invisible fluorescigenous light, and the location of the surface cracks is revealed by the emmission of visible fluorescent light by the penetrant dye which was retained in the cracks after the penetrant composition was removed from the surface of the part.
For best efficiency, particularly for the detection and location of minute surface cracks, as well as intermediate size and gross cracks, it is necessary that the dye penetrant composition have high sensitivity.
Volatile type solvents are commonly employed for extending or thinning dye penetrant inspection solutions or compositions. This is done chiefly for the purpose of lowering the viscosity of the penetrant in order to adapt it for application in spraying systems. Thus for example solvents such as kerosene, light fuel oils, light mineral oil, mineral thinner and methyl ethyl ketone, all highly volatile solvents, have heretofore been employed in prior art dye penetrants. See for example U.S. Pat. Nos. 2,806,959 and 3,429,826. Further, most dye penetrant solutions in practice generally require the use of a combination of solvents, including primary and secondary solvents, extender solvents and wetting agents.
Further, with the advent of new alloys employed in aircraft construction, such as titanium and nickel alloys, considerable effort has been made in selecting solvents which can be compatible with such alloys. Solvents compatible with these alloys generally are selected because of their unusually low chloride content and low sulfur content.
However, the use of volatile solvents in dye penetrant compositions has certain disadvantages. Thus, the use of a volatile solvent in dye penetrants results in the evolution of fumes and solvent vapors which are rapidly formed by the evaporating solvent. Also, when such solvents evaporate, the performance of such penetrants radically changes by changing the washability and sensitivity of the penetrants.
An additional criterion has recently developed also with respect to dye penetrant solutions and compositions. Generally, dye penetrant solutions presently being used and containing solvents and wetting agents present a disposal problem and hence the necessity for the development of dye penetrant solutions and compositions which are biodegradable, that is which employ dye solvents and carriers which are biodegradable, and are readily available despite the petrochemical shortage, has attained considerable importance. At present water pollution laws are becoming more and more strict with respect to the amount of pollutants which can be discarded into sewers and waterways.
Accordingly, in my Patent Nos. 3,915,884 and 3,915,886 there are disclosed water washable biodegradable dye penetrant compositions containing as primary vehicle for the dye, certain nonionic aliphatic oxyalkylated alcohol surfactants, preferably in the form of mixtures thereof.
However, biodegradable penetrants, e.g. those noted in the last two mentioned patents and which contain as primary vehicle the above noted surfactants, have relatively high viscosity. The result is that there is a high "drag out" loss when parts dipped into the dye penetrant solution, as is often done in practice, are removed from the solution. Such "drag out" loss is due to an entrapment of excess penetrant on the parts.
Further, due to the high viscosity of such biodegradable dye penetrant compositions, when such compositions are employed in spraying systems such as electrostatic spraying, the addition of conventional organic solvents such as those noted above for thinning purposes, essentially destroys the biodegradability of the resulting solvent extended penetrant, and/or tends to produce toxic fumes, or increases the flammability, thus defeating the use and advantages gained by employment of an initially biodegradable dye penetrant.
My U.S. Pat. No. 4,186,304 discloses a liquid dye penetrant composition comprising an oxalkylated aliphatic alcohol nonionic surfactant of the types described in the aforementioned U.S. Pat. Nos. 3,915,885 and 3,915,886, and containing as an extender the high boiling narrow-cut isoparaffins having a chain length of about 10 to about 17 carbon atoms. Although the use of such isoparaffinic solvent extender has certain important advantages, including high flash point, low volatility, absence of odor and provides a dye penetrant of low viscosity, the incorporation of such organic solvent in the oxyalkylated aliphatic alcohol surfactant employed as vehicle in the dye penetrant, has the disadvantage of rendering the resulting dye penetrant nonbiodegradable.
My U.S. Pat. No. 3,838,160 discloses that pyrrolidones have been used as a primary vehicle in dye penetrant compositions. The patent discloses also that in such compositions, secondary solvents such as certain ketones can be employed, e.g. isobutyl heptyl ketone. Also there can be incorporated into such dye penetrant composition containing pyrrolidones as primary vehicle, certain water soluble and water insoluble surfactants, such as a nonyl phenyl ether of polyethylene glycol. It is noted that such surfactants are generally used in substantially larger amounts than the primary pyrrolidone dye vehicle in the patent. Thus, the patent discloses nonbiodegradable dye penetrant compositions wherein the volume of pyrrolidone employed as primary vehicle in the patent is essentially maintained minimal for the purpose of dissolving the dye.
Accordingly, an object of the present invention is the provision of an improved dye penetrant composition containing an extender which has a number of unique advantages over extenders previously employed in dye penetrant compositions. A particular object of the invention is the provision of a novel biodegradable dye penetrant composition containing a biodegradable extender which is relatively inexpensive and which is compatible with certain dye penetrant composition vehicles and with certain alloys used in aircraft construction such as titanium and nickel alloys, and particularly affording a biodegradable dye penetrant composition of extremely low volatility, low viscosity, low toxicity and which is substantially odorless. A further object is to provide an extended penetrant employing a non-volatile solvent which, since it does not evaporate, does not change the sensitivity and washability performance of the penetrant.